The sad story of the HTTP referrer and referer
Wednesday, March 19th, 2008
As Google Fight shows, referer is more common on the Internet than referrer, even though referrer is the correct spelling.
Wikipedia describes a possible cause:
Referer is a common misspelling of the word referrer. It is so common, in fact, that it made it into the official specification of HTTP – the communication protocol of the World Wide Web – and has therefore become the standard industry spelling when discussing HTTP referers.